1. Field of the Invention
The present invention relates to a mobile communication system and, more particularly, to an interrogation/transponder system for identifying and locating the position of aircraft on or near an airport surface.
2 . Description of the Prior Art
A mobile communication system for delivering communication between a transmitter, such as an interrogator, stationarily mounted in a certain fixed place, and a simple receiver, such as a transponder, mounted on a mobile object, has attracted considerable attention. Such communication systems have application in many fields. One such field of note is the identification and location of the position of aircraft.
Aircraft are typically equipped with SSR transponders. These SSR transponders respond to interrogation messages transmitted by means of a wide-area surveillance fixed station. The response message from the SSR transponder contains aircraft identification information and serves to identify the position and location of the aircraft as it travels from airport to airport. In order to provide uniformity, the SSR transponders must meet internationally-approved specifications.
These SSR transponders have been primarily used to identify the position of aircraft while they are in the air. In operation, an interrogator, which can be located at or near the airport surface, transmits an interrogation signal which is received and acted upon by the SSR transponder in the aircraft. The SSR transponder transmits a response signal which is picked up by a fixed receiver which can also be located near the airport surface. Using the time delay between sending the interrogation signal and receiving the response, the distance of the transponder from the airport can be measured. Using the scan angle information from the interrogator antenna, the azimuth of the aircraft can be determined.
Choices must be faced when selecting the transmit frequency of the interrogation signal. A trade-off must be made between the narrowness of the band signal and the ability of the signal to penetrate adverse weather conditions such as rain. Lower frequency signals are better adapted to penetrate adverse weather conditions. However, such lower frequencies require a much larger antenna to maintain the same beam width angle. Because of the necessity of accommodating adverse weather conditions, current SSR transponder systems sacrifice beam angle precision in order to transmit low frequency signals.
An emerging problem facing aircraft surveillance is the location of aircraft on the airport surface. The prior art SSR transponder systems used to locate aircraft in the air generally do not face the problem of receiving competing signals from separate aircraft. As a result, these systems use wide beam antenna patterns. However, when determining the aircraft position on the airport surface, the aircraft are in much closer proximity, thereby dramatically increasing the chances of receiving conflicting signals from separate aircraft. The wide beam antenna patterns used by the current SSR transponder systems are generally unable to distinguish closely situated aircraft on an airport surface.
Accordingly, there is a need for an improved aircraft surveillance system capable of distinguishing and locating aircraft on an airport surface. Because of the significant costs associated with such communication systems, it is strongly desired that such an improved system utilize as much as possible of the existing SSR transponder systems.